As a technique for performing noncontact bi-directional communication via radio frequency to exchange data for the purpose of identification, Radio Frequency Identification (RFID) is gaining increasingly wide application.
A typical RFID system generally includes two parts, namely an RFID reader and an RFID tag. The RFID tag is located on the object to be identified and is the data carrier in the RFID system. A typical RFID tag includes a microchip that stores data and a coupling element, such as a coiled antenna, for carrying out radio frequency communication with the RFID reader. RFID tags may be either active or passive. Active RFID tags have an on-tag power supply (such as a battery) and can actively send an RF signal for communication, while passive RFID tags obtain all of their power from the interrogation signal of the RFID reader and either reflect or load modulate the RFID reader's signal for communication. Most RFID tags, both passive and active, communicate only when they are interrogated by an RFID reader.
An RFID reader can read data from an RFID tag and/or write data to the RFID tag. A typical RFID reader includes a radio frequency module, a controller, and a coupling element (such as an antenna) to carry out radio frequency communication with an RFID tag. In addition, many RFID readers are fitted with an information reading interface that enables them to communicate their received data to a data processing subsystem, e.g., a database running on a personal computer.
In most RFID systems, an interrogation signal transmitted by an antenna of an RFID reader can be received by a tag within the coverage (also referred to as “RF region” hereinafter) of the antenna. The size of the coverage depends on the operating frequency of the RFID reader and the size of the antenna. When an RFID tag passes by the coverage of the antenna, it can detect the interrogation signal of the reader, and transmit as reply the information or data on the object to be identified stored therein in response to the interrogation signal. The reader identifies the object identified by the RFID tag according to the received reply returned from the RFID tag.
Compared with contemporary or prior identification techniques such as barcode, magnetic card, IC card or the like, RFID bears such advantages as noncontactness, wide operating range, adaptation to hostile environment, ability of identifying mobile objects and the like. Due to these advantages, RFID has been increasingly used in logistics management. However, when RFID is used in logistics management, real-time management is a hard problem in RFID application layer, as shown in FIG. 1.
One important case in real time management is moving sequence real-time detection problem in many applications such as airport baggage handling system, postal sorting system, food sorting system, etc. An example of the moving sequence real-time detection problem is shown in FIG. 2.
Currently, it is difficult to detect individual RFID tag in a moving sequence, because:
1. When an RFID reader transmits a signal to tags, more than one tag can answer the reader simultaneously.
2. The RFID reader can read a number of tags simultaneously. However, the information read is simple and confused in order, as shown in FIG. 3.
3. It is hard to acquire more specific information, such as individual precise time of every tag in the random interval moving sequence.
4. Collision happens when multiple tags enter RF region simultaneously. Collision throws the natural order into confusion completely, which is mainly manifested as                a. State information is unreliable due to lack of internal power source in the tag in the case of passive tags.        b. Tags cannot communicate with each other. This is a special case of the multiple channel access communication issue.        c. Tags have limited memory and computation capabilities. There exists little calculation possible at tags.        d. Existing researches focus on anti-collision technology, which is basically helpless for detecting the correct order of a moving sequence.        
5. Sequence detection efficiency will be a bottleneck as the anti-collision capacity of the reader increases. Current readers can read more than 600 C1 G2 (Class 1 Generation 2) tags per second. However, it will take about tens of milliseconds to read a single tag in real environment for a special RFID reader. That is, the “global scroll” efficiency is less than “inventory” efficiency.
Sequence detection problem is very difficult for existing method. However, it becomes more and more important because there exist a big market opportunity for moving sequence detection in RFID application. However, most of the current moving sequence detection systems are costly and time-consuming. In one hand, for example, the airport baggage handling systems in Beijing Capital Airport handle approximately 110,000 passengers per day averagely. Current average mishandled bags are more than 5 per 1,000 passengers. 1 Every missing or mishandled bag costs averagely at least 500 RMB. Therefore, the cost due to missing baggage is 275,000 RMB per day and 100.375 million RMB per year. On the other hand, it is time-consuming for individual passenger and air company. For example, the S-3000E Tilt-Tray Sorter is the latest generation in a long line of sorting equipment with sorter velocity up to 3.5 m/sec. Suppose baggage-handling speed is 1 piece of baggage per second. Capacity of an Airbus 380 is 600 passengers, and every passenger has 2 pieces of baggage. Thus the handling time is 1200 seconds, i.e., 20 minutes. If we could increase the handling speed by 5 times, i.e., 5 pieces of baggage per second, then the total handling time would be 4 minutes. This means huge saving of time for passengers and air companies.
Moving confused sequence comprises ordered moving pairs that sojourn in an observed region concurrently with a high probability, i.e., the observed value of sequence Seq={B→A} in observed region X is SeqxεX=({A→B} or {B→A}). The confused sequence information has the following characteristics:
a. Collision occurs when objects A and B enter the observed region
b. There exists a short period in which only individual A presents before collision begins
c. There exists a short period in which only individual B presents after collision finishes
d. No precise method to distinguish the bound of single object and multiple objects and control the observation.
e. The interval between objects A and B is uncertain.
Numerous tags can be present in the interrogation area of an RFID reader. A reader in an RFID system can transmit an interrogation message to the tags. Upon receiving the message, all tags send a response back to the reader. If more than one tag responds, their responses will collide in the RF communication channel, and thus cannot be received by reader. The problem of solving this collision is generally referred to as the anti-collision problem, and the ability to solve this is an important ability.
The simplest of all the multi-access procedures is the ALOHA procedure. As soon as a data packet is available it is sent from the tag to the reader. This is a tag-driven stochastic TDMA procedure. The procedure is used exclusively with read-only tags, which generally have to transfer only a small amount of data (serial numbers), this data being sent to the reader in a cyclical sequence. The data transmission time represents only a fraction of the repetition time, so there are relatively long pauses between transmissions. Furthermore, the repetition times for the individual tags differ slightly. There is therefore a certain probability that two tags can transmit their data packets at different times and the data packets will not collide with one another. The time sequence of a data transmission in an ALOHA system is shown in FIG. 4.
Some kinds of slotted Aloha protocol are broadly used as the basic concept of anti-collision method in commercial tag products, for example, ‘I-code’ by PHILIPS, ISO/IEC-18000-6C and so on. The main idea of this algorithm is to speed up the inventorying process by decreasing useless slots, vacant or collided. However, it is helpless to decide the sequence that RFID tags enter RF region because the correct order has been thrown into confusion by the random selection method in Aloha and related anti-collision algorithm.
The existing researches focus on how to read a possible great number of tags in shortest time. It is helpless or even misleading in detecting the correct order of a moving sequence. The purpose of existing researches is shown in FIG. 5.
As described above, existing solutions focus on large-power method for reading large-number tags. Current anti-collision algorithms throw the order of multiple tags into confusion completely. These methods provide approaches to detect multiple tags in a short time. However, the information read merely includes those that bear no relationship with sequence, such as number, crude time, etc.
It can be seen that there is a need for a system and method for practically and efficiently detecting the correct order of moving RFID tags.